What are causes of lower GI bleeding?

Diverticulosis: One of the most common causes of lower GI bleeding. Small out-pockets, or diverticula, form in the wall of the colon (large intestine), usually in a weakened area of the bowel wall. The person may develop several pockets, which are more common in people who have constipation and strain during a bowel movement.

Infectious diarrhea: Some viruses or bacteria can cause damage to the inner lining of the intestines, which can lead to bleeding.

Angiodysplasia: Along with diverticulosis, this is one of the most common causes of lower GI bleeding. Angiodysplasia is a malformation of the blood vessels in the wall of the GI tract. These are most commonly in the large intestine and often bleed. The elderly and people with chronic kidney failure develop the disease most often.

Polyps: Intestinal polyps are noncancerous tumors of the GI tract, occurring mostly in people older than 40 years of age. A small proportion of these polyps may transform into cancer. Colon polyps may bleed rapidly, or they may bleed slowly and go undetected.

Hemorrhoids and fissures: Hemorrhoids are swollen veins in and around the anus. Repeated stretching from straining during bowel movements causes them to bleed. Bleeding from hemorrhoids is usually mild, intermittent, and bright red. Anal fissures, or tears in the anal wall, also may trigger small amounts of bright red bleeding from the anus. Forceful straining during passage of hard stool usually causes such tears, which can be very painful and may require surgery.